In "The Lagoon," how is it that Arsat and the White Man are friends?

In Joseph Conrad's short story "The Lagoon," we have a cross-racial friendship at the heart of the story, but throughout the story, there is very little of background information about the characters in the story. Even the back-story of Arsat's love and escape is very sketchily told. As far as the relation between Arsat and the White Man is concerned, not much has been told. We are only made to know that the two of them came to know each other through some war and Arsat had saved the life of the White Man, when it was endangered in course of the war. So, in that respect, one can see the White Man's journey into the lagoon as a return-trip, driven by his sense of gratitude.